Missed message -> msg_fmt conversion
Missed message -> msg_fmt conversion

We recently changed NovaException's to define `msg_fmt` instead of `message`
on the class. Looks like this exception was missed for some reason.

Fixes bug 1202255

Change-Id: If828bf42fa077665d1d92e08758fbddd342ab626

n0ano/gantt,n0ano/gantt